1. Field of the Invention
This invention relates in general to a gate controlling circuit for a thyristor converter having a plurality of thyristor arms.
2. Description of the Prior Art
Recently, there has been employed a narrow width pulse system wherein a gate pulse having a narrow width is supplied to each gate of a plurality of serially connected thyristors, i.e., thyristor arms therein, only at the time of firing of the thyristors.
However, in the case where one or more of the thyristors connected in series recovers to a reverse blocking state when most of the other thyristors are conductive, the rated voltage which is imposed across the sum of the thyristors will be applied to the recovered thyristors. Under such conditions, the recovered thyristors will be destroyed.
Of course, even if one or more of the thyristors are turned off, if the gate pulse is immediately supplied or generated, since the turned-off thyristors, i.e., recovered thyristors, can be turned on, the recovered thyristors will not be destroyed. However, in the prior art system one or more of the thyristors are selected from the thyristor arm at random to detect the forward and the reverse bias voltage across the thyristor arm. Therefore, if all of these randomly selected thyristors are in the conductive state it is impossible to detect the forward bias voltage of thyristor arm. As the result, the gate pulse is not produced.
If the voltages across the thyristors connected in series are each individually detected, the whole construction becomes prohibitively expensive. Additionally, such prior art detection systems have been costly to produce because the high voltages present in the thyristor arm necessitates the use of high voltage resistant parts in the detection circuit.